The present invention relates to thermally adjustable optical fiber grating devices, and, in particular, to such devices packaged for enhanced performance.
Optical fibers are key components in modern telecommunication systems. Basically, an optical fiber is a thin strand of glass capable of transmitting optical signals containing a large amount of information over long distances with very low loss. In its simplest form, it is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical do optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical gratings are important elements for selectively controlling specific wavelengths of light within optical systems such as optical communication systems. Such gratings include Bragg gratings and long period gratings. Gratings typically comprise a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves. The ability to dynamically modify these gratings would be highly useful.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, including a plurality of perturbations substantially equally spaced along the waveguide length. These perturbations selectively reflect light of wavelength xcex equal to twice the spacing xcex9 between successive perturbations times the effective refractive index, i.e. xcex=2neffxcex9, where xcex is the vacuum wavelength and neff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
Waveguide Bragg gratings are conventionally fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g. germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source, e.g. an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
In conventional Bragg gratings the dispersion and reflective properties are static. Each grating selectively reflects only light in a narrow bandwidth centered around mxcex=2neffxcex9, where m=1,2,3 . . . is the order of the grating. However for many applications, it is desirable to have gratings which can be controllably altered in center wavelength, bandwith and/or dispersion.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. It typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance xcex9xe2x80x2 which is large compared to the wavelength xcex of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing xcex9xe2x80x2 which is typically at least 10 times larger than the transmitted wavelength, i.e. xcex9xe2x80x2xe2x89xa710xcex. Typically xcex9xe2x80x2 is in the range 15-1500 micrometers, and the width of a perturbation is in the range ⅕xcex9xe2x80x2 to ⅘xcex9xe2x80x2. In some applications, such as chirped gratings, the spacing xcex9xe2x80x2 can vary along the length of the grating. Long-period gratings are particularly useful for equalizing amplifier gain at different wavelengths of an optical communications system. See, for example, U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995.
Thermally adjustable optical fiber gratings are promising elements for optical communication systems. Electrically controlled heating elements in thermal contact with the fiber vary the fiber temperature, producing variations in both the index of refraction and the spacing between successive perturbations. The heating can be uniform along the length of the grating to adjust the center wavelength or it can vary along the grating to adjust the bandwidth and/or dispersion of the grating. Optical grating devices with thermally adjustable bandwidth are described, for example, in the aforementioned U.S. patent application Ser. No. 09/183,048, now U.S. Pat. No. 6,275,629, which is incorporated herein by reference.
In several important optical communications applications near ideal grating performance is critical. For example, potential applications using thermally adjustable gratings for dispersion compensation require a linear response. Conventional adjustable gratings exhibit instabilities and nonuniform heating which fall short of the performance needed for critical applications. Accordingly there is a need for enhancing the performance of thermally adjustable gratings.
This invention is predicated upon applicants"" discovery that the performance of thermally adjustable fiber grating devices is enhanced by disposing them within a vessel for thermal isolation. The vessel is sufficiently larger than the fiber to avoid contact with the grating yet sufficiently small to isolate the grating from substantial air currents. Conveniently, the vessel is a cylindrical tube having elastomeric end seals. Advantageously microcapillary tubes passing through the elastomeric seals provide openings for the fiber to pass through the tube.